The biochemical processes that prevent the aerobic growth of obligately anaerobic bacteria are not understood. Previous theories have failed to adequately explain obligate anaerobiosis for all anaerobes apparently because there is more than one contributing factor. This proposal seeks to identify the mechanisms involved in aero-intolerance of clinically important anaerobes, primarily those involved in oral infection. The role of cytotoxic intermediates of oxygen reduction will be studied in superoxide dismutase (SOD) and peroxidase mutants of Streptococcus faecalis and in catalase mutants of Escherichia coli. Investigations with mutants of these facultative anaerobes will provide information on the function of these enzymes in protecting against oxygen toxicity, and will serve as models for studies with strict anaerobes. Oxygen sensitivity of selected obligate anaerobes will be examined and related to levels of SOD, catalase and peroxidase. Oxygen sensitivity will also be compared to the unproductive drainage of the anaerobes' reducing power (i.e., NAD (P)H), consumption of oxygen and production of superoxide radicals and hydrogen peroxide by respiring cells. Mutants of these anaerobes with increased aerotolerance will be isolated and studied to determine what changes in the above properties are responsible for the decreased oxygen sensitivity. By identifying the processes that provide the mutants with greater protection, it is possible to deduce the mechanisms of oxygen toxicity in the obligate anaerobes.